Speckle-pattern interferometry applied to the study of phase objects

If a speckle pattern is photographed through a phase object, the individual speckles are displaced over varying distances depending upon the phase irregularities in the object. A double exposure is made, the object being present during one of the two exposures only. When the processed photographic plate is observed through an aperture stop in the Fourier transform plane, interference fringes representing contours of the optical path gradient are obtained. The sensitivity of the method and the direction along which the gradient is measured can be varied by modifying, respectively, the distance of the Fourier-plane aperture from the optical axis and its azimuth.     

Laser speckle velocimeter using a pair of virtually shifting photodiodes

A new method for measuring the magnitude and direction of the velocity of a diffuse object is proposed. The method is based on the Doppler effect using the virtual shift of a pair of photodetectors placed on a plane in the far-field of the speckle pattern.     

Dynamic laser speckles and their application to velocity measurements of the diffuse object

In connection with the parameters of the optical configuration used to produce speckles and under illumination of a Gaussian beam over a moving diffuse object, this paper discusses the statistical properties of dynamic speckles produced in the diffraction and image fields by the diffuse object moving in a plane with constant velocity. Especially, the space-time correlation function of the dynamic speckle intensity is examined in detail. Two typical speckle motions of translation and boiling are interpreted for various optical configurations in the diffraction and image fields. As one of the interesting applications of dynamic speckles to metrology, various methods for velocity measurements of the diffuse object are introduced.     

Vibration analysis by laser speckle correlation

Time-averaged laser speckle correlation is used for flexural vibration-studies. The proposed method employs a diverging laser beam to illuminate the test object and records the object image at a defocused plane with a CCD camera. The speckle correlation coefficient is a function of the out-of-plane tilt of the object. The time-averaged speckle pattern recorded with the object vibrating at a natural mode is subtracted from the one recorded in a static condition. The anti-nodes of the vibrating object can then be identified easily on the subtracted image. A brief analysis of the technique and some experimental results on a cantilever beam and a vibrating plate are presented.     

Velocity measurement using the time-space cross-correlation of speckle patterns

The vector velocity measurement of the in-plane object motion based on the dynamic properties of speckle patterns is conducted. The time-space cross-correlation function of speckle intensity variations is used to determine the magnitude and direction of the in-plane object velocity. Theoretical and experimental studies of the speckle correlation technique for the determination of the in-plane object velocity are discussed and the vector velocities of the diffuse object are displayed as a two-dimensional image.     

Measuring velocity of speckle fields with digital speckle pattern interferometry

We investigate a digital method for detecting the velocity of a diffusing object. The technique is based on Digital Speckle Pattern Interferometry (DSPI). A set of reference fringes is generated externally through the reference beam in a digital interferometer. As the object moves, subsequent frames are acquired and subtracted according to the normal DSPI procedure and stored. By means of the theory of first order speckle statistics applied to speckle intensity correlation, we relate the visibility variations in the reference fringes with the object velocity. Thus, by measuring the fringe visibility variation in the resulting DSPI stored frames the mean object velocity can be obtained. The theoretical results are experimentally verified.     

Velocity measurement by the laser speckle method using optical fibres

An optical device for in-plane velocity measurement is described. Laser light is focused by the use of an optical fibre onto the object, and the device utilizes the dynamic sparkle effect for velocity determination. The influence of different optical fibres on the speckle formation is investigated with special respect to fibre movement, and as the diameter of the aperture used for speckle detection also affects the measuring signal, an optimum value is found. A simple electronic circuit processes the measuring value proportional to the object velocity.     

Speckle reduction by a rotating aperture at the fourier transform plane

Statistical properties of the time-averaged speckle-reduced pattern at the image plane of a coherently illuminated uniform diffuse object are theoretically and experimentally studied. The diffuse object is assumed to be approximated by a deep random phase screen and zero-mean circular complex Gaussian statistics is also assumed to hold for the formation of speckles. The time-averaging process is performed by using a rotating aperture at the Fourier transform plane of the object. A theoretical study of the speckle reduction is conducted for a rotating Gaussian soft aperture at the Fourier transform plane. Expressions are analytically derived for the mean, autocorrelation function, power spectrum, and contrast of the time-averaged speckle pattern at the image plane. In particular, the contrast of the time-averaged speckle pattern is theoretically evaluated and its characteristics are discussed in detail. The theoretical results for the contrast are experimentally confirmed using a TV/film recording system. Excellent agreement between theoretical and experimental results is obtained.     

Measurements of axial displacements undergone by a diffusing object in speckle photography

The speckles of the image plane of an object are both radially shifted and decorrelated when the object is axially translated through an amount ?. We demonstrate that this radial shift, which is related to the position of the pupil of the optical system, disappears when the pupil lies in the back focal plane of the imaging lens. However, if the irradiance of the image plane is twice recorded on a photographic plate which is laterally shifted through ζ_H between the exposures, the minimum value of the contrast of the Fourier fringes exhibited by the plate after processing, gives the value of ? if ? is less than a particular value, ?_M which will be defined. Also we propose a new speckle measurement method in which the fringes are automatically removed when ? is greater than ?_M. We record the image of the object illuminated in convergent light through an amplitude diffuser placed in the Fourier plane of the object. The mean speckle speckle size of the diffuser is equal to the mean size of the speckles generated by the object in its Fourier plane.     

Radial and rotational slope contours in speckle shear interferometry

Radial and rotational slope contours are obtained using speckle shear interferometry and a dual refractive index immersion method. Unlike speckle shear interferometry which compares the deformed state of any object with its normal state and thus brings out the structural defects, the method reported here displays prominently any departures from an otherwise regular profile.     

成像型任意反射面速度干涉仪的散斑抑制

成像型任意反射面速度干涉仪(VISAR)是激光驱动聚变实验中诊断冲击波速度的重要设备。由于采用了激光照明靶面的方式,所获得的速度条纹图中不可避免有激光散斑的干扰,严重影响动态条纹的质量。介绍了该系统的激光散斑形成原因和散斑对速度分析的影响,提出了一种频谱面滤波的方式去除高频散斑噪声的方法,并通过搭建散斑光路、合理设置滤波孔位置和大小,对该方法进行了验证。结果表明,该方法对影响条纹图的高频散斑噪声具有抑制作用,适合应用于成像型VISAR系统。     

Effect of surface roughness on the statistical distribution of image speckle intensity

An experimental investigation is made of the effect of surface roughness on the statistical distribution of image speckle intensity. A speckle pattern is formed by spatially coherent light at the image plane of an object having some surface roughness and its statistical properties are investigated. It is found that the contrast of the speckle is related to the surface roughness. By this relation, a new technique for the measurement of surface roughness is proposed.     

Optical velocimetry: a method using two shifted speckle patterns

A new optical method for measuring the lateral velocity of a diffuse object is presented. The speckle pattern, originated by the surface under coherent illumination, is imaged into a linear photodiode array. The scanning time of the array is selected to detect two slightly shifted speckle patterns. By processing the intensity distributions, the velocity of the surface is derived. Some experimental results are shown.     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

Reduction of speckle in an image by a moving aperture - second order statistics

In an earlier paper the second-order statistics of time-averaged speckle were investigated for the case of speckle formed in the Fraunhofer plane when an aperture was moved over the surface of a uniformly illuminated diffuse object. In this paper it is the intention to substantiate a claim that was made in that paper that the same treatment will also apply to time-averaged speckle in an image plane when an aperture is moved in the pupil plane of the imaging lens. The optical system is allowed to have aberrations of any degree of severity, and the object is allowed to have non-uniform brightness, subject to the condition that the brightness variations are slow in comparison to the size of the (aberrated) point spread function. The treatment makes use of the usual assumption that a very large number of independent scattering points in the object contributes to any one point in the image: for this assumption to be realistic, however, it will be shown that the point spread function of the lens must be of a size that restricts us either to systems where the numerical aperture in object space is low or, alternatively, to systems where the aberrations are severe.     

Velocity measurement of the diffuse object based on time-differentiated speckle intensity fluctuations

This paper proposes a new velocity measuring method for a moving diffuse object on the basis of the autocorrelation of time-differentiated speckle intensity fluctuations. The theoretical background and experimental verification of this method are given. The correlation length of time-differentiated speckle intensity fluctuations is defined by the point at which the correlation becomes zero. By means of the correlation length the object velocity is found to be accurately measured.     

基于迈克尔逊干涉的傅里叶变换散斑形貌测量技术

提出了电子散斑干涉载频调制测量物体形貌的方法。采用典型的迈克尔逊干涉光路,将物体偏转一微小角度（等效为物面与参考面间形成空气楔）产生等厚干涉,可在物体的表面引入包含物体高度信息的载波干涉条纹。用CCD采集该载波条纹图,利用傅里叶变换法可解调出物体高度的位相信息,从而实现物体的形貌测量。介绍了电子散斑干涉载频调制测量物体形貌的原理,并进行了实物测量,给出了实验结果。由于该方法采用散斑干涉方法测量物体形貌,所以具有灵敏度高的优点。